Disclosed is an optical semiconductor device having an optical semiconductor mounting element including a recess. On an inner side face of the recess is a thermosetting resin composition for light reflection, which, after curing, can realize high reflectance in a range of visible light to near ultraviolet light, has excellent heat deterioration resistance and tablet moldability, and is less likely to cause burrs during transfer molding, and a process for producing the resin composition, and an optical semiconductor element mounting substrate and an optical semiconductor device using the resin composition. The heat curable resin composition for light reflection comprises a heat curable component and a white pigment and is characterized in that the length of burrs caused upon transfer molding under conditions of molding temperature of 100° C. to 200° C., molding pressure of not more than 20 MPa, and molding time of 60 to 120 sec is not more than 5 mm and the light reflectance after heat curing at a wavelength of 350 nm to 800 nm is not less than 80%.